Isoflavones are crystalline ketones found primarily in leguminous plants. One of the most important sources of isoflavones is the soybean, which contains twelve distinct isoflavones: genistein, genistin, 6″-O-malonylgenistin,6″-O-acetylgenistin, daidzein, daidzin, 6″-O-malonyldaidzin,6″-O-acetyidaidzin, glycitein, glycitin, 6″-O-malonylglycitin, and 6″-O-acetylglycitin (Kudou, Agric. Biol. Chem. 55, 2227-2233, 1991). These soybean isoflavones share the generic structure shown below:
                where R1═H, OH, or OCH3;        
and                R3═H, CH3C(O) or HOOCCH2C(O).        
Dietary isoflavones are believed to have health benefits. For example, they are believed to be responsible for the cholesterol-lowering effect of soy products and may help prevent breast cancer. Moreover, isoflavones are believed to ameliorate menopausal symptoms. U.S. Pat. No. 5,972,995 teaches the treatment of cystic fibrosis patients by administering isoflavones capable to stimulate chloride transport.
Soy protein isolates are typically prepared from defatted soy meal. Proteins and soluble carbohydrates are extracted into aqueous solution at about pH 7-10. The insoluble residue is mostly fiber and is removed by centrifugation. The protein is precipitated from solution as curd at its isoelectric point (about pH 4.5), further purified, neutralized, and dried. The liquid remaining after the protein has been isolated is referred to as whey and contains mainly soluble carbohydrates. Most of the isoflavones are retrieved with the protein curd.
Isoflavones also exist at the parts per million (ppm) level in the whey. Soy whey also contains carbohydrates, primarily sugars such as the monosaccharides glucose and fructose and the oligosaccharides sucrose (disaccharide), raffinose (trisaccharide) and stachyose (tetrasaccharide), in addition to proteins, salts and other bioactives. The oligosaccharides raffinose and stachyose require the enzyme α-galactosidase, which is not present in the human gastrointestinal tract, to be completely hydrolyzed into monosaccharides that can be absorbed into the blood stream. Unhydrolyzed oligosaccharides consumed by humans pass into the large intestine where they are fermented by anaerobic microorganisms producing gases such as CO2, H2, and CH4 that lead to flatulence.
Currently, the soy whey is treated as waste, resulting in significant disposal costs. A process for treating soy whey, and other biological or plant processing waste products, to recover the isoflavones and to remove the undesired oligosaccharides, to give a solution of digestible sugars for use in food products, would therefore be highly desirable.
Methods for recovering isoflavones from soy whey are known in the art. For example, a process for separating specific isoflavone fractions from soy whey and soy molasses feed streams is described in U.S. Pat. Nos. 6,033,714; 5,792,503; and 5,702,752. In another method, soy molasses (also referred to as soy solubles) is obtained when vacuum distillation removes the ethanol from an aqueous ethanol extract of defatted soy meal. The feed stream is heated to a temperature chosen according to the specific solubility of the desired isoflavone fraction. The stream is then passed through an ultrafiltration membrane, which allows isoflavone molecules below a maximum molecular weight to permeate. The permeate then may be concentrated using a reverse osmosis membrane. The concentrated stream is then put through a resin adsorption process using at least one liquid chromatography column to further separate the fractions.
Amberlite XAD-4 polymeric adsorbent (Rohm and Haas, Philadelphia, Pa.) is described in U.S. Pat. No. 6,033,714 as particularly attractive for use in the chromatography column. XAD-4 has been described as a hydrophobic, crosslinked styrene/divinylbenzene polymer [Kunin, Polym. Sci. and Eng., 17(1), 58-62 (1977)]. XAD-4 has good stability and its characteristic pore size distribution makes it suitable for adsorption of organic substances of relatively low molecular weight. As disclosed in U.S. Pat. No. 6,033,714, however, other adsorptive resins may be used in the chromatography column.
In another method, U.S. Pat. No. 6,261,565 describes a composition, enriched in isoflavones, that is obtained by fractionating a plant source high in isoflavones, including soy molasses and soy whey. In that process, the aqueous solution containing the isoflavones is passed through an ultrafiltration membrane and then fed through a resin column to isolate the isoflavones.
In all these disclosures, a polymeric adsorbent is used to recover the isoflavones from an aqueous mixture. However, in order to recover the low level of isoflavones in plant processing waste products, such as soy whey, more effectively, an adsorbent with a higher affinity for isoflavones is required.
Methods for the removal of oligosaccharides from soybean wastes are also known in the art. For example, Matsubara et al [Biosci. Biotech. Biochem. 60:421 (1996)] describe a method for recovering soybean oligosaccharides from steamed soybean wastewater using reverse osmosis and nanofiltration membranes. JP 07-082,287 teaches the recovery of oligosaccharides from soybean oligosaccharide syrup using solvent extraction. That method comprises adding an organic solvent to the aqueous solution containing the oligosaccharides, heating the mixture to give a homogeneous solution, cooling the solution to form two liquid layers, and separating and recovering the bottom layer. However, these methods are not selective for the removal of the undesired oligosaccharides raffinose and stachyose, which are recovered along with the desirable sugars, sucrose, glucose, and fructose.
A method for recovering isoflavones and separating oligosaccharides from bean curd waste solution is described in KR 2000/055,133. In that method, the waste solution is passed through a polymeric resin column to remove saponin and isoflavones. Then, the waste solution that passed through the column is filtered and concentrated to recover the oligosaccharides. However, the undesirable oligosaccharides raffinose and stachyose are recovered along with the desirable sugars, sucrose, glucose, and fructose. There have been no reports in the art of a process for recovering isoflavones and selectively removing the undesired oligosaccharides raffinose and stachyose from plant processing wastes.
Zeolites are high capacity, selective adsorbents that have been widely used for separating a variety of chemical compounds. Zeolites can be generically described as complex aluminosilicates characterized by three-dimensional framework structures enclosing cavities occupied by ions and water molecules, all of which can move with significant freedom within the zeolite matrix (Meier et al, Atlas of Zeolite Structure Types, Elsevier, 2001). In commercially useful zeolites, the water molecules can be removed from or replaced within the framework structures without destroying the zeolite's geometry.
Zeolites have been widely used as bulk adsorbents and as chromatography supports for separating a variety of substances including gases, hydrocarbons and alcohols. The use of zeolites as selective adsorbents for carbohydrates is well known in the art. For example, the use of zeolites for the separation of monosaccharides is described by Ho et al [Ind. Eng. Chem. Res. 26:1407 (1987)], and Sherman et al [Stud. Surf. Sci. Catal. 28:1025 (1980)]. Additionally, a process for separating monosaccharides using zeolite adsorbents is described in U.S. Pat. Nos. 4,405,377 and 4,483,980. The adsorption, selectivity of the zeolites to monosaccharides is determined by the extent of interaction with the cations present in the zeolite, as well as the geometric constraints imposed by the zeolite pore geometries and cation positions, as discussed by Sherman et al. [Stud. Surf. Sci. Catal. 28:1025 (1980)].
Buttersack et al [J. Phys. Chem. 97:11861 (1993)] report that the dealumination of Y-zeolites enhance their affinity to mono-, di-, and trisaccharides by hydrophobic interactions. The adsorption of oligosaccharides such as raffinose and stachyose by a hydrophobic zeolite, specifically dealuminated FAU type zeolite (Si/Al=110), is described by Buttersack et al [Langmuir 12:3101 (1996)]. The FAU type zeolite used in that investigation was sold by Degussa Company, South Plainfield, N.J., under the product name Wessalith® DAY-55. That disclosure reports that DAY-55 has a very strong affinity for stachyose, a strong affinity for raffinose and sucrose, and very little affinity for glucose when tested in a single component system. The adsorption characteristics of DAY-55 were not tested in a multi-component system consisting of a mixture of sugars.
A need remains for a simple, economical process to recover isoflavones and remove undesirable oligosaccharides such as raffinose and stachyose from aqueous mixtures such as soy whey and other plant processing waste products. Such a process would yield several useful products including isoflavones, and a solution containing digestible sugars, such as glucose, fructose, and sucrose.